Breakdown of Мне не удалось отказать клиенту.
не
not
я
I
клиент
client
удаться
to manage / to succeed (perfective)
Questions & Answers about Мне не удалось отказать клиенту.
Why is клиенту in the Dative case?
The verb отказать (to refuse or deny) requires the Dative case without a preposition for the person being refused. Just like the verbs of assistance you are learning (such as помогать), the action is directed to a receiver. Think of it literally as "to say no to someone."
Why does the sentence start with Мне instead of Я?
The verb удаться (to manage or succeed) requires an impersonal construction. Instead of saying "I managed," Russian phrases it as "It succeeded to me." Because of this, the logical subject "I" must be in the Dative case, which becomes Мне.
Why does удалось have the neuter ending -ось?
In an impersonal sentence where an infinitive verb (like отказать) serves as the main subject of the action, the past tense verb always takes the default neuter singular ending. Since удаться is a reflexive verb, it gets the neuter reflexive ending -ось.
Could I use the imperfective отказывать instead of отказать here?
You need the perfective отказать here because you are talking about the result of a single, specific event (you failed to refuse the client this one time). You would only use the imperfective отказывать if you were talking about a repeated failure to refuse the client over an ongoing period of time.